Internal frost recording



May 2, 1967 L. F. BEAN ETAL 3,317,316 INTERNAL FROST RECORDING Filed May 17, 1963 5 Sheets-Sheet l INVENTORS LLOYD F. BEAN ROBERT W. GUNDLACH A TTORNE V INTERNAL FROST RECORDING Filed May 17, 1963 3 Sheets-Sheet 2 L2 FC- 43 FLUOROCARBON F22 DENSITY 8 SILICONE OIL D0200 (PROJECTED r4 IMAGE) CARBOWAX 400 .4- r .2 STAYBELITE o I I I ESTER IO LO I.| L2 L3 L4 L5 REFRACTIVE INDEX (RELATIVE TO AIR) INVENTORS LLOYD F. BEAN F 3 BY ROBERT w. GUNDLACH ATTORNEY May 2, 1967 P BEAN Em. 3,317,316

INTERNAL FROST RECORDTNG Filed May 17, 1963 5 Sheets-Sheet Z SYNCHRONIZATIQN 5 35 INVENTORS LLOYD F. BEAN ROBERT W. GUNDLACH AT TORNE) United States Patent 3,317,316 INTERNAL FROST RECORDING Lloyd F. Bean, Rochester, and Robert W. Gundlach,

Victor, N.Y., assignors to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed May 17, 1963, Ser. No. 281,233 2 Claims. (Cl. 96-1.1)

This invention relates to electrostatic frost recording and in particular to frost deformation at an interface of multilayered structures. Electrostatic deformation recording is a relatively recent art having its origins in the last decade when it was discovered that a latent electrostatic image could produce a fairly high resolution deformation in a low viscosity material. These early deformation processes had severe limitations in that they showed no promise of continuous tone capabilities and were very difiicult to utilize usually requiring complex optical projection systems to convert the deformation pattern into images of contrasting light characteristics.

A major advance in the art of electrostatic deformation was made more recently when it was discovered that with certain limitations as to materials and process steps it was possible to obtain a deformation image having the finely diffuse characteristics commonly seen in frosted glass and the like. This new type of electrostatic deformation imaging has been termed frost deformation and is particularly valuable in producing continuous tones in an immediately visible and useable image as the result of the light contrast between light diffusing areas, i.e. the frost areas, and the background areas which are non-light diffusing. Thus, by electrostatic frost imaging, the first really good continuous tone electrostatically formed images have been produced. A further advantage of this process is the lack of any necessity for developing material to be added for visible imaging. Since frostable materials are readily adaptable to flexible web or tape structures, it is immedi ately obvious that here might be a suitable medium for image recording on continuous tapes, or reels of tapes where the image may be stored and used at will. Of further interest along this line is the fact that such frost images are readily erased by heating so that the frostable member becomes reuseable much in the fashion that a magnetic tape is reuseable, after erasure, for new recordings. However, in attempting to adapt frostable materials for repeated use and reuse, a major difiiculty has been encountered in the tendency for these materials to pick up dust and foreign matter from the atmosphere deteriorating the performance of the material and producing defects in the images formed. This problem becomes more apparent on consideration that in the usual frost material it is necessary to soften the material by heating or solvent action to permit deformation. This softening or heating generally produces a tacky condition which captures and holds lint and other foreign matter so as to prevent any effective cleaning. Additionally, if a tape of frostable material is to be wound on a reel for storage purposes with or without images, it is necessary that the frostable layer be firm and non-tacky while being wound and while being stored so as to prevent frostable material on one part of the tape or web from sticking to the back of the tape on some other part of the web. This means that the materials used for processing must have a high viscosity and a relatively nontacky surface when wound on a reel and while being stored. Thus, after an image is formed by softening the material, the material must be adequately hardened before winding. This prevents the use of low viscosity ma terials that never completely harden to a dry, non-tacky surface, if they are to be wound and stored on a reel. Likewise, reels cannot be stored under conditions in which the frostable materials can reach a softened state.

3,317,316 Patented May 2, 1967 Now in accordance with the present invention, it has been discovered that frostable materials can be made and used in a way such that it is possible to continuously reuse materials without any deterioration in imaging due to foreign matter collecting on the frostable surface. Further in accordance with the present invention, it has been found possible to use materials which tend to remain in a relatively tacky or low viscosity state and to wind the reels while still in such a state and even to store tapes carrying frost images under conditions causing the frostable material to soften and become tacky. These advantages have been achieved by discovering tech niques for forming frost images internally of a laminated structure so that the frost image, as well as the frostable layer or layers, is not exposed to the outside atmosphere or to the contact of other adjacent materials such as might be encountered in the process of image formation, image projection, or in winding on a reel. Thus, it is an object of the invention to define a deformation imaging member for forming frost images at an interface. It is an additional object of the invention to define a process for forming a frost image at an interface. It is still a further object of the invention to define methods and apparatus for reuseable frost imaging. Further objects and features of the invention will become apparent while reading the following description in connection with the drawings wherein like numerals are used to indicate like elements throughout.

FIGURE 1 is a diagrammatic illustration of an embodiment of the invention using corona charging.

FIGURES 2, 3, and 4 are diagrammatic illustrations of the embodiments of the invention using conductive electrode charging.

FIGURE 5 is a diagrammatic illustration of the embodiment of the invention using selective charging in accordance with a pattern to be reproduced.

FIGURE 6 is a diagrammatic illustration of a continuous frost imaging system.

FIGURE 7 is a diagrammatic illustration of a frost imaging system using reeled web.

FIGURE 8 is a graph showing the relation of projected image density to the refractive index of the frost materials.

Frost imaging, as disclosed in patent application Ser. No. 193,277, now Patent No. 3,196,001, filed May 8, 1962, is a process in which an outside surface of a thermoplastic layer is selectively subjected to electrostatic stress forces and then softened so that the stressed areas wrinkle to a fine grained light diffusing texture. A similar type of frost image, in accordance with the present invention, can be formed at an inside surface more properly denominated an interface. Looking at FIG. 1 for example, a frost image may be formed at the interface of insulating thermoplastic layer 11 and low viscosity. conductive layer 12 in the multilayered member 10. Member 10 further comprises insulating photoconductive layer 13, first supporting layer 15, second supporting layer 16 and conductive layer 17. For reasons that will be discussed, it is preferable that each of these layers be substantially transparent.

Supporting layers 15 and 16 can be made of a tough, hard-surfaced electrically insulating material such as glass. Where flexibility is desired, a plastic resin such as polyethylene terephthalate or other polyethylene material or acrylic plastic can be used. Any other insulating transparent material capable of maintaining dimensional stability to temperatures of up to about F. or higher and preferably about 200 F. is suitable. The necessary thermal stability will be determined to some extent by the heat that will be applied to develop or erase the frost image since the requirement is that the supporting plastic layers remain essentially undistorted by such heating. A conductive layer 17 is coated on transparent supporting member 15. This conductive layer must be such as to enable an electrically conductive connection to be made to it. Since this layer should be transparent, one of the common transparent conducting layers such as tin oxide or copper iodide is suitable. These layers may be applied in any of the ways known to the art such as, for example, evaporation. The thickness of the layer is not critical. Second supporting layer 16 of the same nature as layer 15 is coated with an insulating transparent photoconductive material. This insulating photoconductive material may suitably be an organic photoconductor such as one of those described in Canadian Patents 568,707 and 611,852 to Neugebauer et'al. and assigned to Kalle & Company. Dip coating, whirl coating, spray coating or other coating process such as described in the said Canadian patents is suitable. The thickness of the photoconductive insulating layers is suitably about 2 to 5 microns and is operable in a range of about one-half micron to 25 microns. In the embodiment of FIG. 1, as will be apparent in discussing the theory of operation below, it is not essential that the photoconductive layer remain insulating when heated. Normally in the dark, and without the application of heat for development or erasure, the photoconductive layer should have a resistivity of 10 ohm. cm. or more, the most critical feature being a virtual absence of lateral conductivity. Over the photoconductive layer, frostable layer 11 is coated. The frostable layer is suitably a thermoplastic or other readily softenable material that is normally electrically insulating and firm at room temperatures. A partial listing of suitable materials may be found in the aforementioned U.S. patent application, Ser. No. 193,277. Particularly suitable materials for layer 11 in the embodiment of FIGURE 1 have been found to be Staybelite ester 5 and Staybelite ester 10 available from the Hercules Powder Company and Pliolite (Type S-7) available from Goodyear Tire and Rubber Company. The frostable material should be softenable to the viscosity of about 10 to 10 poises at temperatures between 100 F. and 200 F. The preferred temperature range is determined at the lower limit by the necessity of maintaining stability under storage and readout conditions and at the high end by the desire to use as little heat as possible in the interests of efficiency as well as by the requirement of staying within the thermal range that will produce distortion or other deterioration in the other layers. The frostable layer 11 is coated over the photoconductive layer 13 by a conventional coating procedure as previously described such as dip, whirl, or spray coating. As will be discussed below, the thickness of this layer in FIGURE 1 should be substantially less than that of the photoconductive layer. Thinner layers generally .give better resolution than thicker layers, but maximum image density starts falling off below about one micron thickness. Between frostable layer 11 and conductive layer 17, layer 12 is applied of material that is relatively electrically conductive as compared to the material of photoconductive layer 13 when in the dark and preferably having a resistivity less than 10 ohm-cm. since higher resistivities tend to decrease the exposure speed of the member. Layer 12 should be of low viscosity or at least softenable, by the temperature required to reduce the viscosity of layer 11 to that required for frost, to a viscosity preferably of the same order or lower than that of layer 11 when heated to frost temperature. Suitable materials have been found to be fluids such as water, alcohol, glycerin, sucrose acetate isobutyrate or a nonionic detergent such as Glim available from B. T. Babbitt Inc. of New York city and materials solid at room temperature such as certain solid polyethylene glycols as, for instance, some available from the Carbide and Carbon Chemicals Company under the name Carbowax and soft or readily softenable plastic or petroleum base materials suitable for frostable layer 11 or various waxy materials such as paraffin to which a conductivity agent has been added. While many additives can produce the necessary conductivity, a particularly appropriate one for plastic resin materials that will not reduce the transparencies of the material has been found to be stannic chloride. Insulating liquids may also be used with the addition of conductivity agents. These liquid materials suitably being silicone oil such as DC200 silicone oil (Dow Corning Corp. of Midland, Mich.), petroleum oil or the like material which may be selected to have a viscosity under the operating conditions for the member that will enable maintaining the desired thickness for layer 12 without introduction of substantial non-uniformity in thickness under the various manipulations encountered in the operative process. Another suitable material has been found to be polymerized ethylene imine 50% aqueous solution having molecular weight range of 30,000 to 40,000 available from Chemirad Corporation of East Brunswick, NJ.

Embodiment of FIG. 1 is adapted for applications using corona charging and has the particularly desirable characteristic that when the latent electrostatic image is formed, it is trapped on highly insulating surfaces so that electrical conductivity produced in most transparent photoconductive materials by heating is not objectionable and such photoconductive material may be used. Thus, in the operation of FIGURE 1, the member 10 is placed in the dark and a corona discharge device 18 is used to charge the surface of transparent supporting layer 16 to a voltage of about 100 to 2000 volts. This charging voltage may be either positive or negative, but is illustrated in FIGURE 1 as a positive voltage which is applied with reference to conductive layer 17. For this purpose, conductive layer 17 is suitably electrically connected to ground or to a reference side of the power supply for operating corona discharge device 18. After charging, member 10 is exposed to a light image of a pattern to be reproduced. For purposes of illustrative simplicity, this is diagrammatically shown as using crosshatching 20 as representative of dark-areas in a projected image. After the exposure, charge device 18 brings the surface of transparent support layer 16 back to a uniform potential. The exposure maybe made through either transparent support layer 16 or through transparent support layer 15 and, if desired, charge may be simultaneous with exposure so that separate charging before and after exposure is not necessary. After the charging and exposure steps, member 10 is heated until a frost image appears at the interface of frostable layer 11 and relatively conductive layer 12. The temperature required will vary with the amount of electrical charge applied, to the extent of exposure and the thermal characteristics of frostable layer 11. The further factor involving the frost threshold is the relative surface tension of the frostable material layer 11 and the relatively conductive material of layer 12, since frost is apparently associated with surface tension eifects. Selection of the material for these two layers to reduce surface tension will likewise reduce the frost threshold so that a frost image may be developed with higher viscosities, lower electrical fields or both. Some interfacial tension is necessary to enable erasure; however, most nonmiscible materials seem to provide adequate tension. For example, surface tension can be reduced considerably by using a material such as Staybelite ester 5 for layer 11 and silicone oil, Dow Corning Type DC 200, for layer 12 with conductivity agents added as .necessary. The heat required to develop a frost image will generally be in the range of 100 F. to 200 F. and must be applied until the viscosity of layer 11 is reduced to about the range of 10 to 10 poises. After the frost image is formed, member 10 is cooled to fix the image and then the image may be observed by light transmitted through the member. It can be observed directly by light passing through the opposite side of the member or with a projection lamp and lenses. The frost image may be focused on a projection screen. Since the transparent supporting layers and 16 preferably have no lightdiffusing characteristics, the light-diffusing image may be viewed from the same side that the member is illuminated from, that is, the side on which all layers are transparent down to the image interface, but such a diffusely reflecting image is relatively inefficient in its use of light. Thus, transmitted light is preferable for projection purposes and, accordingly, it is desirable that all layers of the imaging member be transparent.

As is obvious, since the frost image produces equivalent deformations in both the frostable layer 11 and the relatively conducting layer 12, the deformations would not be visible if these materials had identical refractive indexes. Accordingly, in all cases a substantial difference in refractive index must exist between the materials on each side of the frostable interface so that the frost image will diffuse intercepted light. To provide adequate image density without special optics, a difference in absolute refractive index of .2 or more is considered preferable. However, smaller differences in refractive index are acceptable when a Schlieren or similar type of optical system can be used to enhance contrast.

While not to be considered limiting, the theory of the present invention as it is believed to operate in the embodiment of FIGURE 1 may be simply stated. When the corona discharge device is first operated under the surface of layer 16, it applies a uniform density of electrostatic charges as indicated by a first row of signs illustrated immediately adjacent to the surface of layer 16. In response to the field generated by the corona discharge device and the charges applied to the surface of layer 16, charges of opposite polarity passing through the connection to reference potential are attracted through conductive layer 17 and relatively conductive layer 12 to the interface of the relatively conductive layer 12 and insulating frostable layer 11. These latter charges are indicated by a first row of signs immediately adjacent to the interface of layers 11 and 12. It will be seen that the charges applied to the surface of layer 16 are trapped by the insulating layer 16 and cannot readily migrate elsewhere. Now on exposure to a light image, the electrical charges innate to photoconductive layer 13 become relatively mobile in the illuminated areas. Under this condition, in the illuminated areas, charges of one polarity on the surface of layer 16 attract mobile charges of the opposite polarity from the bulk of photoconductive layer 13 to the interface of layer 13 and the support layer 16. Likewise, in these areas, charges on the surface of layer 16 repel opposite polarity mobile charges from the bulk of photoconductive layer 13 to the interface of layer 13 and the insulating layer 11. This movement of charges lowers the voltage appearing between the surface of layer 16 and the interface of layers 11 and 12. Looking at it another way, member 11 can be viewed as a capacitor in which the plates are considered to be the surface of layer 16 on one side and relatively conductive layer 12 on the other side with a dielectric between them comprised of layers 11, 13 and 16. The capacity of this capacitor will be determined by the formula K is the dielectric constant of the layers, A is the area of the plates and d is the thickness of the dielectric. Upon exposure to light, the exposed areas of photoconductive layer 13 become relatively conductive, reducing the effective thickness of the dielectric and thus increasing the capacity as is obvious from the above formula. With charged capacitors, the voltage across the capacitor will vary with the capacity providing the charge is held constant as may be seen in the formula Q=CV in which Q=the charge in coulombs, C=the capacity in farads, V=potential in volts. In the present case, the charge is trapped on the surface of layer where C is the capacity,

16 and cannot vary. Thus, when the capacity is increased by decreasing the effective dielectric thickness on illumination of the photoconductor, the voltage across the assembly is reduced in the illuminated areas. On recharging, the corona discharge device restores the voltage uniformly to the same level as before and, in doing so, increases the charge density in the illuminated areas shown by a second row of signs above the first row at the surface of layer 16 and by a second row of signs below the first row at the interface of layers 11 and 12. On heating member 10 to develop the frost image at the interface of layers 11 and 12, the entire photoconductive layer may become conductive (as long as it remains laterally insulating) without any noticeable loss of image response, since this will only decrease the voltage across the combined member and will not effect the charge distribution. Note that frost development is produced by variations in charge density, unlike xerographic powder development dependent on voltage variations.

In an arrangement much more critical as to the photoconductive material, the relatively conductive layer illustrated as layer 12, FIG. 1, can be eliminated, and a frost image can be formed at the interface between the frostable insulating layer 11 and deformable photoconductive layer. (Thus, in FIG. 1, a slight modification can be made using a deformable photoconductor. The relatively conducting layer 12 may be eliminated and insulating layer 11 may be bonded directly to conductive layer 17.) Appropriate photoconductive materials for this purpose can be obtained by mixing transparent organic photoconductive materials, such as those listed in the aforementioned Canadian patents assigned to Kalle & Company, in solutions containing appropriate mixtures of insulating and readily softenable resins such as, for example, polyvinyl chloride and Staybelite in order to give the desired low viscosity on heating and still be highly electrically insulating in the absence of light. Heat and pressure is usually adequate to accomplish bonding between preformed layers. Although with the use of appropriate materials and process steps to avoid miscibility of the layers in the coating procedure, it is possible to start with one of the supporting layers and coat each of the succeeding layers consecutively upon it. I

With such a deformable insulating photoconductive layer, it is also possible to carry out variations of the inventive concept without the use of corona discharge devices. In these variations, the further limitation is placed upon the deformable insulating photoconductor in that it must remain highly insulating in the unexposed areas when the member is heated to develop the frost image. An embodiment of these variations is illustrated in FIG- URE 2. In the embodiment of FIGURE 2, the conductive layer 17 is coated on transparent supporting layer 15 as in FIGURE 1. On the conductive layer 17, a deformable photoconductive layer 25' is coated by any conventional technique such as dip coating, whirl coating, or spray coating or, where a layer of elemental photoconductor is used, coating by evaporation. This deformable photoconductive layer can be made of transparent organic photoconductive material mixed with polyvinyl chloride and Staybelite as suggested above. However, the combination of resins used must be selected so as to avoid excessive loss of resistivity by heating. Thus, the resistivity, in the presence of the heating required to develop a frost image and in the absence of light, of the resin-photoconductor material must be 10 ohm-cm. or greater. Insulating deformable layer 11 is coated on the deformable photoconductive layer and support layer 16 precoated with second transparent conductive layer 26 is applied with the conductive side first against the surface of deformable insulating layer 11.

For operation, a voltage source is connected between conductive layers 26 and 17. The polarity of this connection is not critical; however, some photoconductive layers are more sensitive under a given polarity of applied voltage and, in such cases, the polarity producing the greatest sensitivity is used. The voltage source may appropriately be a pulse generator capable of delivering an electrical pulse for a predetermined interval of time. The member is exposed to a light image of a pattern to be reproduced as for example light reflected from subject 27 through lens system 28 and focused on deformable photoconductive layer 25 of the member. The exposure is made simultaneously with the member being heated to frost temperature by heating device 31 (illustrated in FIGURE 3) which is appropriately an electrical resistance heating element or any other conventional heating source for such purposes and with the voltage pulse application. Exposure, heating and application of the voltage may be conducted until the frost image is formed or they may be applied for a fixed interval as determined by experimentation. The applied voltage is operable within the range of 350 to 1200 volts, and the necessary heating, as with the other embodiments, should be in the range of 100 F. to 200 F., depending upon the temperature necessary to soften the photoconductive insulating material and the deformable insulating material to a viscosity of about 10 poises such as is necessary to produce frost.

The operation of the embodiment of FIGURE. 2 is somewhat different from that of FIGURE 1. For frost deformation, the surface or interface being deformed must be subjected to a build-up of electrical charges immediately at the interface. In FIGURE 2, after the application of voltage, no charges appear at the interface of layers 11 and 25 in the dark. On illumination to an image pattern, layer 25 becomes relatively conducting in the illuminated areas, and charges migrate to the interface providing the necessary frost conditions in those areas.

In this embodiment, the exposure and heat the application of the potential,

for development is preferably simultaneous since this will maintain different conductivities and, hence, different charge in the illuminated areas with respect to the non-illuminated areas. The heat for development may be applied before exposure or during exposure as desired as long as the deformable layers are at frost temperature at some time during the exposure and while the member is in a charge condition.

By replacing the deformable insulating layer 11 of FIGURE 2 With deformable conducting layer 12 and inverting conductive layer 17 to place it at the external surface of transparent support layer 15, we have an embodiment which will frost in the non-illuminated areas as illustrated in FIG. 3. Embodiment of FIG. 3 may be fabricated applying a transparent conductive layer such as copper iodide to transparent supporting layer 15 of A mil polyethylene terephthalate for example. The copper iodide or similar material is suitably evaporated on transparent support layer 15. The other surface of transparent support layer 15 is then coated with deformable photoconductive layer 25 substantially identical to layer 25 in FIG. 2. Nonmiscible transparent conductive deformable layer 12 is then coated over photoconductive layer 25. Second transparent supporting layer 16 similar to layer 15 is then coated with transparent conductive layer 26 of the same nature as conductive layer 17. The coated transparent supporting layer 16 is then applied with the coated surface against the surface of deformable conductive layer 12 and bonded by pressure or heat and pressure. The operation and theory of operation in the embodiment of FIGURE 3 is that on application of potential source 27, electrical charges migrate uniformly through layer 12 to the deformable interface. On exposure to a pattern of illumination, the charges in the illuminated areas migrate on through the photoconductor removing the charge from the deformable interface in the illuminated areas. On heating, only the non-illuminated areas frost.

In embodiments where the photoconductor mustremain insulating in the presence of heat, a greater flexibility exists in the choice of photoconductive materials if they are not required also to be deformable. Thus, FIGURE 4 illustrates an embodiment that is essentially similar to FIGURE 2, except that the need for a deformable photoconductive material has been avoided. Instead of the deformable photoconductive layer 25 of FIGURE 2, nondeformable photoconductive layer 13, similar to layer 13 in FIGURE 1, is utilized. The photoconductive material of layer 13 in FIGURE 4 is slightly more limited than that used in FIGURE 1 since it must remain insulating although heated to frost temperatures. However, it may be readily made as from a solution of an organic photoconductor mixed with polyvinyl chloride or other plastic resin that does not readily exhibit conductivity on heating. Insulating deformable layer 11 is then coated over the photocondutcor as in FIGURE 2, and a relatively conductive thermoplastic layer essentially identical to layer 12 in FIGURE 1 is coated on top of the deformable insulating layer 11. This relatively conductive layer 12 forms a deformable interface with deformable insulating layer 11.

A preferred example of the embodiment of FIGURE 4 comprises a 5 microns organic photoconductor for layer 13, 2 /2 microns of Staybelite ester 10 for layer 11, and about 1 micron of fluorocarbon for layer 12 and /2 mil Mylar having evaporated coatings of copper iodide for layers 15 and 16. Layer 13 can be made, for example, by mixing TO 1920 photoconductor available from Kalle & Company and Vinylite VYNS resin available from Carbide and Carbon Chemicals Co., along with Rhodamine B dye and dimethyl ethyl ketone as a solvent. Exemplary proportions are 10 grams TO 1920; 10 grams VYNS; 1 milligram Rhodamine B and 50 milliliters of dimethyl ethyl ketone. This mixture is dip coated over transparent electrode 17 and then baked for three hours at C. This baking process reduces lateral conductivity improving resolution. The Staybelite ester 10 dissolved in Super Naphtholite (American Mineral Spirits Co.) is dip coated over layer 13, and a further baking of one-half hour at 50 C. evaporates the solvent. Layer 12 is then formed by placing on Staybelite layer 11 a small amount of FC 43 fluorocarbon available from Minnesota Mining and Manufacturing Co. Layer 16 carrying transparent electrode 26 is then applied electrode side first against the fluorocarbon which spreads out into a uniform layer. The voltage applied by source 27 should be about 350 to 1200 volts. Dielectric breakdown becomes a hazard over 1000 volts; however, the higher voltages give greater electrostatic contrast and enable the use of harder materials or shorter development.

Examples in accordance with the embodiment of FIG- URE 4 have produced resolution greater than 100 lines/mm. with no relief edges. Previous frost images have shown relief edges and frost resolution under 50 lines/ mm. is substantially greater than previously achieved by frost techniques. The resolution improvement is believed attributable to the fact that the image charges in this embodiment are bound in place at a non-deformable interface while the induced charges are at the deformable interface. It does not appear that this configuration allows tangential vectors of electrostatic force at the location of deformation. In prior art frost configurations, the image charges are bound at the deformable surface so that tangential vectors of electrostatic force affect the deformation. This high resolution also applies to the embodiments of FIGURES 1 and 5.

The operation and theory of the embodiment of FIG- URE 4 is identical to that of FIGURE 2, except that electrical charges will migrate through relatively conducting layer 12 to the interface of layers 11 and 12 under the potential applied from source 27 and as influenced by the dielectric presented by photoconductive layer 13 and insulating layer 11.

In the embodiments of FIGURES 1 and 4, the capacitor analogy given in the suggested theory of operation of FIG- URE 1 is applicable. In both these embodiments, electrical charges can move to the deformable interface through relatively conductive layer 12. Thus, on the application of potential, the deformable interface receives electrical charges distributed evenly over its entirety. In the formation of the latent charge image, it is then necessary to distribute additional charge in accordance with the image configuration so that the desired image areas will frost first. The image density will depend on the relative amount of this additional charge distribution as compared to evenly distributed charge. The amount of such additional charge is in turn dependent on the increase in capacity attained as determined by the previously stated formula 7 can be seen that photoconductive layer 13 preferably occupies a disproportionate amount of the total electrical thickness of the combined dielectric in order to enable a maximum change in capacity on illumination. This is best accomplished by using material of a low dielectric constant (K) for layer 13 and material of a high dielectric constant for layer 11 and in FIGURE 1 for layer 16 also. The physical thickness of the layers is then adjusted so that the equivalent electrical thickness. (-d/K) of the photoconductive layer in the dark is at least as great, and desirably 2 to 3 times greater, than that of layers 11 and 16 combined in FIGURE 1 and layer 11 alone in FIGURE 4.

In both of these embodiments, it is also desirable that insulating layer 11 have a thickness of /2 to 5 microns, and preferably 2 to 3 microns. Thinner layers give higher resolution, and thicker layers enable greater density.

In the embodiments of FIGURES 2 and 3, the thickness of the deformable layers is less critical, but it is desirable that at least one of the deformable layers be kept within the range of /2 to 5 microns to provide good resolution.

In still further variations of the present invention, interfacial frost can be obtained in members having no photoconductive layers. FIGURE 5, for example, illustrates an embodiment in which stencil 29 is positioned over an insulating exterior surface of the member, and corona charging is used to deposit electrostatic charge on the surface in a pattern corresponding to the openings inthe stencil. A member for this purpose may be made in embodiments that are similar to the embodiments of FIG- URES 1-4 with the difference that no photoconductive material is used, and all layers on the side of the deformable interface on which the surface to be selectively charged lies must be highly insulating to avoid shielding effects. Accordingly, FIGURE 5 shows a transparent plastic support layer 15 coated first with a transparent conductive layer 17, secondly with a relatively conductive deformable layer 12. This layer, as with all the other layers designated 12 in the application, need be only relatively conductive as compared to the usual insulating photoconductive layers in the dark; that is, having less resistance than about ohm-cm. Also, it may be a liquid or a softenable material that is solid at room temperature. A third coating of deformable transparent insulating material 11 is applied over the relatively conductive material, and this material, as with the material in layer 11 in other embodiments, must be highly insulating having resistivities of 10 ohm-cm. or higher even when heated to frost temperatures of 100 F. to 200 F. On deformable insulating layer 11, a fourth coating with transparent insulating material 16 having a non-deformable surface at frost temperatures and similar to support layer 15 is applied.

In operation, corona discharge device 18 charges insulating support layer 16 through stencil 29 so as to deposit charge on the surface of layer 16 through openings 30. In response to the electrical charges placed on the surface of layer 16, charges of opposite polarity, passing through the connection of transparent conductive layer 17 to ground or a reference potential, migrate through the deformable conductive layer 12 to areas at the interface of layers 11 and 12 corresponding to those areas where charge has been deposited on the surface of layer 16. Heating to form a frost image may be simultaneous with charging or can be a separate step after charging. Since the insulating layer 16 will maintain the charge configuration for an extended period of time, development may be delayed for such an extended period.

In FIGURE 5, the layer thicknesses are not particularly critical; however, the insulating layer 11 is suitably in the range of A2 to 5 microns for good resolution.

The present invention is particularly applicable in com bination with apparatus in which the imaging member is to be continuously recycled or is to be stored for a length of time carrying images on it which may be later erased in order to reuse the frostable material at a later time. Apparatus for continuous recycling is illustrated in FIGURE 6. The frostable member in FIGURE 6 is illustrated in the form of a cylindrical drum 40. While the frostable member for this purpose may be any of those previously described, it is illustrated as similar to that in FIGURE 2 and like numerals indicate like elements of the FIGURE 2 illustration. Thus, transparent support member 15 in the apparatus of FIGURE 6 is preferably a rigid glass or transparent plastic member in the form of a cylinder which carries the other layers in accordance with the invention. The cylindrical drum 40 is rotatable by a motor 4l'through a sequence of processing stations in the operation of the apparatus. These operating stations comprise an exposure or recording station 45 illustrated as a projector for projecting an image pattern of light and shadow onto imaging member 40. Following exposure station 45 in the direction of drum rotation is a development station 48 which is suitably an electrical resistance heating element, an infrared heating element or other convenient form of heat applicator for raising the imaging member to frost temperature. After developing station 48 is readout station 50. Readout station 50 is illustrated as an optical projection system for transmitting light through imaging member 40 and focusing it in image configuration on a screen or on second recording member 51. Thus, the readout system illustrated comprises projection lamp 52, reflector or light shield 53, condensing lens 55 and projection lens 56. Continuing in the direction of rotation, erasing station 57 follows the readout station. Using an imaging member of the nature of that shown in FIGURE 2, the erasing station suitably comprises heat source 59. Heat source 59 suitably comprises electrical resistance heating elements that do not radiate actinic light. When the imaging member is an endless member operating in cyclical fashion as in the embodiment of FIGURE 6, the conductive layers 17 and 26 are preferably connected by a short circuit during erasure. This ensures restoration of the imaging member to an electrically neutral condition. As illustrated in FIGURE 6, this may be accomplished by dividing layer 26 into three segments separated by insulatingdividers 32. (Such insulating dividers in the case of a tin oxide layer are readily provided by etching away a strip of the layer. A subsequently applied plastic layer will fill in any space.)

In the vicinity of exposure station 45 and development station 48, brush contact 42 and brush contact 46 connect potential source 43 across the conductive layers 17 and 26. Note that brush contact 42 is a double contact to enable electrical connection across insulating dividers 32 in layer 26 and to ensure potential application during exposure and development. The erasure station is positioned opposite the center of brush contact 42 to enable maximum separation between the position where potential is applied and the position for the short circuit. The short circuit is applied by brush contact 33 connected to a common reference potential 34 with brush contact 46. The frost image is relatively stable by the time it reaches readout station 50, and the potential applied at that position is not necessarily critical. Due to the segmentation of layer 26, it is necessary to synchronize the drum rotation with the exposure system to avoid straddling a divider 32 with an exposure frame. Synchronization can be by any known device for that purpose as indicated by block 35.

In the operation of the apparatus in FIGURE 6, the imaging member 40 is rotated by motor 41, so that it passes through exposure station 45 where an image is focused on the imaging member 40 as by a flash exposure system in which the flashes are fast enough to effectively stop the movement of the drum. This exposure permits charge migration in the photoconductive layer in accordance with the imagepattern so that the charge density in layers 17 and 26 also varies in accordance with the image pattern to maintain the potential constant. At development station 48, the imaging member is then heated until a frost image appears at the interface of layers 11 and 12. After development, the frost image passes through readout station 50 where a light from light source 52 passing through imaging member 40 is modulated by the frost image and is focused by projection lens 56 as an image pattern of light and shadow on screen or recording member 51. As in exposure station 45, light source 52 in the readout station can be flashed at a speed adequate to effectively halt the movement of the drum. After the frost image has been utilized at readout station 50, it is erased at erasure station 57 by the simultaneous application of a higher degree of heat than was used for image development and a short circuit between conductive layers 26 and 17. The heat softens the deformable layers while the short circuit permits electrical discharge of the electrostatic image. With the deformable layers softened and discharged, interfacial tension of the deformable materials causes erasure of the frost. In the rotation from the erasing station 57 to the exposure station 45, the deformable layers are cooled and are again ready for a new imaging cycle.

FIGURE 7 shows an apparatus in which the same processing stations as in FIGURE 6 are designated by like numerals. However, a flexible imaging member or web 60 is used, mounted on a supply reel 61 and fed to a storage reel 62, and corona charging is used instead of electrode layers. Unlike the imaging member 40 of FIG- URE 6, transparent support layer of flexible member 60 cannot be rigid, but is preferably a flexible plastic material such as polyvinyl chloride, polyethylene terephthalate and polytetrafluoroethylene. This web can be generally of the structure illustrated in FIGURE 1, having one transparent conductive layer connected at reel 61 to electrical reference 63. In the apparatus of FIGURE 8, the imaging member 60 is drawn from supply reel 61 through a charging station 65, exposure station 45, recharging station 66 and developing station 48. After development at station 48, the imaging member 60 is then drawn through a cooling station 67 where heat sinks or refrigerated plates positioned adjacent to the path of travel of the imaging member cool the web and freeze the frost image. The imaging web 60 is wound on storage reel 62, rotated by motor 70 where the web carrying the frost images may be stored for an indefinite period before utilization. The images on this stored web may be readout at any time in a device essentially similar to a movie projector. The web carrying the images may be utilized repeatedly in projecting the same images again and again or, when the particular stored im ges are 110 longer desired, the images may be erased by heating and the web reused in a new imaging process.

In projecting the deformation images of the present invention, the difference in refractive index of the materials at the deformed interface determine the maximum available image density. FIGURE 8 illustrates the difference in refractive index obtained using a series of materials, each forming an interface with Staybelite ester 10. The size of the projection lens opening must be considered, along with refractive index, to determine the projected image density. The smaller the lens opening, the greater the density, but the poorer the resolution and light efficiency. Generally, a lens opening of less than f8 is considered too small for microphotographic use. Accordingly, referring to FIGURE 8, it will be seen that materials with a refractive index greater than water when used with Staybelite in the invention will operate best with the use of a Schlieren projection system or the like, particularly for microphotography.

The imaging member and apparatus of the present invention is of particular value when used in connection with computer readout and facsimile readout systems or in making black-and-white films for television use where a large quantity of imaging material is used, and/or it is desirable to avoid the delay necessary for photographic film processing. The desirability of the present imaging system and material where large quantities of imaging material are required is in the reusable feature which enables repeated reuse of the same material without the expense of replacing it after every use.

While the present invention has been described as carried out in specific embodiments thereof, there is no desire to be limited thereby, but it is intended to cover the invention broadly Within the spirit and scope of the appended claims.

What is claimed is:

1. The method of forming an image consisting of light diffusing discontinuities in an otherwise optically transparent member comprising:

(a) depositing on electrostatic charge by means of a corona discharge device on the surface of a first insulating layer of an integral member comprising:

(1) a first insulating layer of a transparent thermally stable material;

(2) a transparent photoconductive layer;

(3) an electrically insulating deformable layer;

(4) a deformable conductive layer having a substantially different refractive index from said deformable layer; and,

(5) a second conductively coated insulating layer of transparent thermally stable material;

(b) exposing said member to an image pattern to be reproduced;

(c) recharging said surface to a uniform potential; and,

(d) heating said member until the interface formed by said insulating deformable layer and said conductive deformable layer deforms in a frost image corresponding to the image to be reproduced.

12. An integral imaging member for forming a frost image pattern at an interface by deforming said interface into a pattern of random light diffusing irregularities corresponding in density to the variations in an image input supplied thereto comprising:

(a) a first insulating support layer of a transparent thermally stable material;

(b) a transparent photoconductive layer contacting said first insulating layer;

(c) an electrically insulating frostable layer positioned in contact with the face of said photoconductive layer non-adjacent said insulating layer;

((1) a deformable conductive layer positioned in contact with said frostable layer and having a substantially different refractive index from said frostable .layer; and

(e) a conductively coated insulating support layer of transparent thermally stable material, the conductive 13 14 coating of said face being positioned in contact With 3,169,061 2/1965 Hudson 340-173 said deformable conductive layer. 3,196,008 7/1965 Mihajlov et a1 34674 X 3,196,013 7/1965 Walkup 96-1 References Cited by the Examiner UNITED STATES PATENTS 5 BERNARD KONICK, Primary Examiner. 2,896,507 7/1959 Mast et a1. 96-1 J. BREIMAYER, Assistant Examiner. 3,069,681 12/1962 Sloan 34674 

2. AN INTEGRAL IMAGING MEMBER FOR FORMING A FROST IMAGE PATTERN AT AN INTERFACE BY DEFORMING SAID INTERFACE INTO A PATTERN OF RANDOM LIGHT DIFFUSING IRREGULARTIES CORRESPONDING IN DENSITY TO THE VARIATIONS IN AN IMAGE INPUT SUPPLIED THERETO COMPRISING: (A) A FIRST INSULATING SUPPORT LAYER OF A TRANSPARENT THERMALLY STABLE MATERIAL; (B) A TRANSPARENT PHOTOCONDUCTIVE LAYER CONTACTING SAID FIRST INSULATING LAYER; (C) AN ELECTRICALLY INSULATING FROSTABLE LAYER POSITIONED IN CONTACT WITH THE FACE OF SAID PHOTOCONDUCTIVE LAYER NON-ADJACENT SAID INSULATING LAYER; (D) A DEFORMABLE CONDUCTIVE LAYER POSITIONED IN CONTACT WITH SAID FROSTABLE LAYER AND HAVING A SUBSTANTIALLY DIFFERENT REFRACTIVE INDEX FOR SAID FROSTABLE LAYER; AND (E) A CONDUCTIVELY COATED INSULATING SUPPORT LAYER OF TRANSPARENT THERMALLY STABLE MATEIAL, THE CONDUCTIVE COATING OF SAID FACE BEING POSITIONED IN CONTACT WITH SAID DEFORMABLE CONDUCTIVE LAYER. 